The invention concerns an apparatus for processing body signals, which includes a sensor for picking up in particular electrical signals from a living body, and means for preparing picked-up signals for further processing, and at least one first memory for a picked-up measurement signal or signal portions.
It is frequently a matter of concern to identify given features in for example electrically recorded signals. For example, it may be a matter of interest to identify T-waves or QRS-complexes in an electrocardiogram and to determine as accurately as possible the time at which they occur. To analyse such signals, it is desirable to locate certain events or signal features, from the point of view of time.
With that background in mind, the present invention is concerned in particular with picking up intracardially recorded signals, in particular in an implanted device. For reasons relating to power requirement and the limited amount of space involved, only restricted resources for signal processing and analysis are available in an implanted device.
Various apparatuses and methods of feature identification of heart signals and for signal analysis are already known. European patent No 0 487 429 for example discloses an apparatus in which is stored a sequence of values of such parameters which correspond to an active cardiac cycle. The apparatus includes comparison devices to compare the stored parameter values of the active cardiac cycle to previously stored items of information about those values and, in the event of a positive comparison result, to trigger a signal. German patent applications Nos 199 38 376 and 199 63 246 which are not prior publications also concern apparatuses in which a measured cardiac signal is compared to previously formed and stored comparison signals in order in the case of DE 199 38 376 to identify fusion events in the electrostimulation of the heart and in the case of DE 199 63 246 to detect the circulatory effect of extrasystoles.
U.S. Pat. No. 5,439,483 also discloses the use of a wavelet transform for the classification of tachycardias. In U.S. Pat. No. 5,778,881 a wavelet transform is additionally combined with a hidden Markov modelling in order to be able to detect P-R-waves in each case as Markov states with a reduced number of wavelet coefficients. It is further proposed therein that a set of wavelet coefficients, which is typical for the respective result, can be automatically updated in the event of rapid changes in the signal morphology, in order to make the analysis independent of short-term fluctuations in the physiological signals for example due to physical stress.
The aim of the present invention, in physiological signals such as intracardial electrocardiograms (ECG) and intracardial dynamic impedance patterns (IDZ), is to determine the occurrence of certain features and the moments in time thereof, hereinafter also referred to as time location, in an efficient manner. That should be effected as reliably as possible even in the presence of noise and interference signals. The degree of accuracy of time location of a feature is to correspond to the sampling rate, that is to say the time raster with which the physiological signals are recorded. Feature recognition is also to function in the event of a variation in signal morphology.
Further aims of the invention concern improved feature analysis functions.
Therefore the object of the present invention is to permit better or more efficient signal analysis in comparison with the state of the art at least in individual areas and thus very substantially to attain the above-indicated aims.
In accordance with the invention, that object is attained by an apparatus of the kind set forth in the opening part of this specification, which includes at least one second memory which contains a predeterminable comparison signal which is finite in respect of time, and signal comparison means which are connected to the second and the first memories and which are adapted for sliding or continuous comparison of signal portions, which overlap in respect of time, of the measurement signal in the first memory to the comparison signal stored in the second memory and for output of a correlation coefficient representing the similarity of each compared signal portion of the measurement signal to the comparison signal.
The integral of the comparison signal in relation to time or its sum of time-discrete signal values is preferably zero. In that way, the correlation signal formed by the correlation coefficients is a signal which is of a mean value of about zero. That greatly simplifies subsequent signal analysis.
In this case, the measurement signal is preferably an intracardial electrogram (ECG) or an intracardially recorded dynamic impedance pattern (IDZ). If those signals are not recorded continuously but in time-discrete manner with a sampling rate, the first memory has a sequence of time-discrete measurement values representing the measurement signal. The comparison signal is also stored in the second memory as a finite sequence of time-discrete values. In that case, instead of the integral of the comparison signal in respect of time preferably its sum of all discrete signal values is zero. That complies with the requirements made in relation to wavelets. Therefore the comparison signal can also be referred to as a comparison wavelet.
A crucial difference in relation to the wavelet transform is that the apparatus does not provide a two-dimensional result, like the wavelet transform, as the comparison pattern is not subjected to time scaling for the investigation of each measurement signal portion over a given frequency range. That decisively reduces the computing power required.
The level of resolution in respect of time of the measurement signal in the first memory is preferably the same as that of the comparison signal in the second memory. Each measurement signal portion to be compared to the comparison signal then corresponds to the comparison signal, in respect of the time duration and the number of discrete measurement values. The correlation coefficients formed by the comparison of the signals also form a sequence of time-discrete values which respectively represent the similarity of precisely one signal portion in the first memory to the comparison signal. The corresponding signal portions of the measurement signal in the first memory preferably overlap in that case in such a way that they are displaced relative to each other only by a discrete time step corresponding to the time resolution of the measurement signal. Due to this sliding comparison, the procedure produces a time-discrete correlation signal which is formed by the correlation coefficients which occur in succession in respect of time, as the result of comparison of the signals. The level of time resolution of the correlation signal is then the same as that of the measurement signal and the comparison signal. The correlation signal however can also be formed with a lower level of time resolution if the comparison operation is implemented only for each second, third or n-th time step. The measurement signal portions which are used for comparison with the comparison signal are then displaced relative to each other from one comparison operation to another comparison operation in each case by two, three or n time steps. In a practical context however it will frequently be advantageous to record the measurement and the comparison signals with a correspondingly lower sampling rate which defines the time raster, if the lower level of time resolution of the signal still permits secure feature identification and reliable signal comparison.
An essential feature of the apparatus is the second memory which contains the comparison signal which can be predetermined in accordance with the respective signal feature to be detected and in particular is variable for adaptation to a varying signal morphology by the apparatus itself.
A preferred apparatus is one in which the signal comparison means are connected to a logarithm storage means or memory which contains tables of logarithms for values of the measurement and the comparison signals, in which respect the signal comparison means are adapted to form the correlation coefficients in such a way that they effect multiplication of a value of the comparison signal from the second memory by the corresponding value of the first measurement signal from the first memory, in such a way that firstly the logarithms of the values themselves to be multiplied or the values which are respectively closest thereto are read out of the logarithm memory and then the two logarithms are added.
A procedure of that kind which is known per se, by the use of logarithm tables or slide rules for the multiplication of two values can be effected in an efficient, memory-saving fashion.
A preferred apparatus further has detection means which are connected to the signal comparison means and which are adapted to detect maximum values and/or zero-passages of a signal formed by the correlation coefficients.
Preferably, the apparatus also has threshold value comparison means which are connected to the signal comparison means and a threshold value memory containing a threshold value and which are adapted to output an identification signal as soon as the correlation coefficient outputted by the signal comparison means exceeds the threshold value. In that case, the apparatus is preferably of such a configuration that the threshold value comparison means are so designed that they output an identification signal when a correlation coefficient for a first signal portion from the first memory exceeds the threshold value and for a second signal portion which is recorded in terms of time after the first signal portion reaches the value zero or is below that value. In addition, there are preferably provided locating means which are connected to the threshold value comparison means and the detection means and are so designed that they associate a location signal with that measurement signal portion in the first memory, for which the signal formed by the correlation coefficient is at a maximum within that section of the signal formed by the correlation coefficient, in respect of which the threshold value comparison means output an identification signal.
As each correlation coefficient of the sequence of correlation coefficients is associated precisely with a signal portion of the measurement signal stored in the first memory, it is possible, by determining the corresponding maxima of the sequence of correlation coefficients, to determine precisely the time location, that is to say the location of a feature in the signal being investigated and thus the time at which a feature occurs.
The apparatus further preferably has threshold value-forming means which are connected to the threshold value memory and the locating means and which are so designed that they form a new threshold value after the occurrence of a location signal in such a way that the correlation coefficient associated with the location signal is involved in a weighted condition in the formation of the new threshold value. That permits continuous adaptation of the threshold value to the actual configuration of the measurement signal and to changes in the morphology thereof.
Also preferred is an apparatus which has comparison signal-forming means for forming a new comparison signal, which are connected to the second memory and which are so designed that a measured signal portion corresponding to a signal feature to be detected is transformed to the comparison signal in such a way that its integral in relation to time or the sum of the time-discrete signal values is zero, and the comparison signal formed in that way is transferred into the second memory. The comparison signal-forming means thus permit automatic formation of a suitable comparison signal.
A preferred apparatus is also one which includes comparison signal-adaptation means for adaptation of the comparison signal, which are connected to the first memory, the second memory and the locating means and which are so designed that they form a new adapted comparison signal when the locating means output a location signal, wherein the adapted comparison signal is formed using that measurement signal portion from the first memory with which the location signal is associated. That permits continuous adaptation of the comparison signal to the actual morphology of the measurement signal, with the consequence that characteristics of the measurement signal are reflected in the comparison signal so that the comparison signal can also be analysed, instead of the measurement signal, for analysis of the measurement signal. Furthermore, that adaptation of the comparison signal permits secure, reliable and time-accurate feature detection.
In this respect the apparatus is preferably distinguished by comparison signal-adaptation means which are so designed that the comparison signal which is valid prior to the adaptation operation, for formation of the comparison signal which is valid after the adaptation operation, is involved, multiplied with a weighting factor of 1-xcex1, in the comparison signal which is to be freshly formed, while that signal portion in the first memory, with which the location signal for triggering adaptation of the comparison signal is associated, is involved with a weighting factor of xcex1 in the comparison signal which is valid after the adaptation operation. In that respect, xcex1 is a value of between 0 and 1. The new comparison signal then corresponds to the sum of the two weighted signals which are involved in the formation of the new comparison signal.
The comparison signal-forming means and/or the comparison signal-adaptation means are preferably also so designed that the comparison signal which is formed or adapted is standardised in such a way that the amplitude thereof corresponds to the maximum amplitude of the measurement signal. That avoids unwanted effects as a consequence of signal multiplication in the formation of the correlation coefficients, which would cause square distortion of the scale for the threshold value.
Preferably the apparatus has a database which contains a plurality of comparison signals and which is connected to the second memory in such a way that comparison signals can be transferred from the database into the second memory and vice-versa. In that way, the apparatus can operate with various comparison signals for the detection of various signal features.
In addition, the apparatus preferably has analysis means which are designed to analyse the characterising properties of the preferably adapted comparison signal. Analysis means of that kind permit analysis of the measurement signal indirectly by evaluation of the comparison signal which is adapted to the measurement signal.
In a preferred variant the apparatus includes means for recording two cardiac signals of which one is associated with the left ventricle or atrium and the other is associated with the right ventricle or atrium, and means connected to said recording means for forming a bimodal signal from the two cardiac signals, in such a way that the bimodal signal contains a feature of the first signal prior to its conduction into the respective other ventricle or atrium and the corresponding feature after its conduction, so that the feature is contained in the bimodal signal at a spacing in respect of time corresponding to the conduction time, on the one hand in its form prior to conduction and on the other hand in its form after conduction. In addition the second memory of that apparatus contains a bimodal comparison signal which can be adapted to the bimodal signal so that, after adaptation of the bimodal comparison signal to the bimodal signal, the conduction time can be determined by analysis of the comparison signal. In the context of the described apparatus, that permits highly accurate determination of the conduction time between two heart chambers.